1. Technical Field
This invention relates to means for driving a brushless D.C. motor, and more particularly to novel means that provide for reversing a D.C. motor using reversible D.C. power such that a brushless D.C. motor may be substituted for a brush-type D.C. motor without changing the means used to control direction of rotation.
2 Background Art
In a brush-type D.C. motor, the field is produced by permanent magnets and is generally stationary, that is, it is located in the stator. The rotor has wound on it windings which are connected to a commutator comprising bars which are sequentially connected to the windings. Brushes which contact the commutator bars are so positioned that the appropriate winding is energized when the rotor has moved to a position such that peak torque will be developed by the interaction of the current in the winding and the permanent magnet field. It is clear that, with such a motor, reversal of the polarity of the applied power will result in a reversal of the rotation of the rotor.
In the case of a brushless D.C. motor, the stator generally is the portion of the motor that includes the windings and the permanent magnet is located in the rotor. This is done for various practical reasons, the most important of which is to enable the leads to be brought out from the motor without going through a commutator, thus a "brushless" D.C. motor. In such a case, the windings are energized sequentially by commutation means which usually consists of a solid state switching circuit, the solid state switches being triggered to the conducting state in a sequential pattern in accordance with the location of the rotor with respect to the stator windings and the desired direction of rotation. Reversing the polarity of the supply voltage would not cause such a motor to reverse; indeed, it would only damage the semiconductor switches because they are unipolar in character. It is therefore necessary to provide for reversal of the motor by a different means and the only means that is known at the present is to change the sequence of winding energization.
The sequence of winding energization is governed by a permanently memorized pattern that is imprinted on a PROM (or its equivalent in discrete components) and the sequence is then governed by the PROM's calling for one switch after the other to be energized. When it is desired to reverse the direction of rotation, it is necessary to command the PROM to determine which of the two sequences it will call on for the switching. To do this, a logical "one" or a logical "zero" command is applied to the portion of the logic circuit that controls the PROM to enable the PROM to apply one of the sequences.
In general, then, direction of rotation of a brush-type D.C. motor is determined by the polarity of the applied power, while direction of rotation of a brushless D.C. motor is determined by a command to a logic circuit, the brushless D.C. motor drive receiving only unidirectional power. It would be desirable in some cases, generally for retrofit purposes, to be able to substitute a brushless D.C. motor for a brush-type D.C. motor in a given application and have the direction of rotation of the brushless D.C. motor be determined by the polarity of the applied power.